1. Field of the Invention
The present invention relates to a microwave and millimeter wave circuit including an amplifier and band elimination filters, and in particular, to a microwave and millimeter wave circuit for use in integrated circuits (referred to as ICs hereinafter) for microwaves, quasi-millimeter waves and millimeter waves at frequencies of about 800 MHz to about 300 GHz.
2. Description of the Prior Art
FIG. 13 is a circuit diagram of a microwave and millimeter wave amplifier circuit of a first prior art.
Referring to FIG. 13, in an amplifier circuit, a gate and a drain of a field effect transistor (referred to as an FET hereinafter) Q having a source grounded (referred to as a source-grounded FET hereinafter) are inserted between an input terminal 1 and an output terminal 2, and between two microstrip lines T11 and T12, each of which is a transmission line. In this amplifier circuit, a band elimination filter (referred to as a BEF hereinafter) FE comprising a series circuit of a microstrip line T13 of a transmission line functioning as an inductor, and a capacitor C11, and having a stopping frequency f.sub.c lower than a desired amplification center frequency f.sub.0 of the amplifier circuit is connected to the drain of the FET Q. This amplifier circuit has a gain frequency characteristic as shown by a gain G10 in FIG. 17. In this case, the amplifier circuit has had such a problem in that there is unnecessary gain, as shown by G11, at lower frequencies near the stopping frequency f.sub.c.
In order to solve this problem, as shown in FIG. 14 showing a second prior art, a directional coupler comprising two 1/4 wavelength microstrip lines T21 and T22 which are opposed to each other and electromagnetically coupled with each other is provided instead of the microstrip line T12, by which the improper gain G11 is eliminated as shown by a gain G12 of FIG. 17.
FIG. 15 is a circuit diagram of a microwave and millimeter wave amplifier circuit of a third prior art as disclosed in the Japanese Patent Laid-Open Publication No. 8-274552.
Referring to FIG. 15, reference numeral 1 denotes an input terminal, 2 denotes an output terminal, 3 and 4 denote bias applying terminals, reference character Q denotes an FET, T11 to T16 denote microstrip lines, which are transmission lines functioning as inductors, C11 and C12 denote capacitors, Rg denotes a resistor, and Ls1 and Ls2 denote high-frequency stopping inductors. A frequency characteristic of the microwave and millimeter wave amplifier circuit constructed as described above has an unnecessary gain arising at about 2 GHz or lower, as shown in FIG. 18, in a manner similar to that of the first prior art. Also, the stabilizing factor K of the circuit is lower than at frequencies of about 13 GHz to 27 GHz, so that the operation of the circuit is unstable.
Further, FIG. 16 is a circuit diagram of a microwave and millimeter wave amplifier circuit of a fourth prior art as disclosed in the U.S. Pat. No. 5,412,347.
Referring to FIG. 16, reference numeral 1 denotes an input terminal, 2 denotes an output terminal, 3 and 4 denote bias applying terminals, reference character Q denotes an FET, T11 to T17 denote microstrip lines, which are transmission lines functioning as inductors, C11 to C15 denote capacitors, and R11 denotes a resistor.
The microwave and millimeter wave amplifier circuit of the second prior art indeed has solved the problem of the first prior art. However, since the microwave and millimeter wave amplifier circuit comprises the directional coupler, the 1/4 wavelength line at 60 GHz, for example, has a length beyond 400 .mu.m. Therefore, the directional coupler is relatively large, and this results in the problem that the circuit including the amplifier circuit can not be miniaturized.
In order to solve the problem of the third prior art, the stabilizing factor K can be made to be equal to or more than one, for example, by adding a microstrip line functioning as a relatively long inductor to the source of the FET Q as shown in FIG. 19. However, there still exists an unnecessary gain. Inserting the inductor, which is a feedback circuit, causes the amplification gain of the amplifier circuit to lower from about 18 dB to about 6 dB with a decrement as much as about 12 dB. Besides, the fourth prior art also has a problem similar to that of the third prior art, so that a larger gain can not be obtained in wider band.